TRANSPIRATION. 117 



mass of evidence to shew that this actually takes place. Thus 

 the very numerous experiments which have been made upon 

 the effect of removing a ring of cortex from stems and 

 branches, although the results are somewhat discordant, tend 

 to shew that the parts above the wound receive sufficient 

 supplies of the salts absorbed by the root, and these supplies 

 must necessarily be conveyed to them by the fibre-vascular 

 tissue. Again, the fact that when coloured solutions are 

 supplied to the root they stain the fibro-vascular tissue, 

 support this conclusion, and it receives further confirmation 

 from Sachs' lithium-experiments, to which allusion was made 

 in the last lecture. 



We have finally to discuss the effect of transpiration in 

 producing a negative pressure of the gases in the vessels 

 of the plant, and the influence of this condition upon the 

 movements of fluids in the plant. 



The first indication of the fact-that, in a transpiring plant, 

 the gases in the vessels are at a lower pressure than that of 

 the atmosphere is to be found in the experiments made by 

 Hales " whereby to find out the force with which trees imbibe 

 moisture." He found that, in a tube connected with a cut-off 

 branch, "the mercury rose highest when the sun was very 

 clear and warm; and towards evening it would subside 3 or 4 

 inches, and rise again the next day as it grew warm." Further 

 indications of the same kind are given in the experiments of 

 Meyen, Sachs and others, but it was von Hohnel who gave 

 the definitive proof. He found that if the stem of a transpiring 

 plant be cut through under mercury, the mercury will at once 

 rise to a height of several centimetres in the vessels, the greatest 

 height being reached in the youngest vessels. This sudden 

 rise of the mercury can only be accounted for by ascribing it 

 to the difference between the atmospheric pressure and that 

 of the gases in the vessels. It also teaches the important fact 

 that the cavities of the vessels of the wood of a plant are not 

 in direct communication with the external air, but that they 

 form a completely closed system. 



An explanation of how this difference of pressure is brought 

 about will perhaps be best given by reference to one particular 



